The purpose of this research is threefold: 1) to verify that plasma levels of opiate receptor ligands are increased during exercise, 2) to determine if the intensity and duration of exercise influence the magnitude of increases in plasma opiate receptor ligands and 3) to determine if changes in these ligands are related to psychological mood state. A 3 x 4 factorial design will be used with levels of intensity at 40, 60 and 80% of maximal oxygen consumption (VO2max) and the duration being 20, 40, 60 and 80 min. The 80% treadmill runs will not be conducted at 60 and 80 min. since these would be beyond the capability of even the best runners. Subjects (N=15) will be experienced male runners between the ages of 20-35 years. Opiate receptor ligands will be determined using a radio-receptor assay according to the general procedures of Nabor et al. 1980. Mood state will also be determined before and after each treadmill run (10 treadmill runs per subject) using the Profile of Mood States. It is our hypothesis that the pain of exercise acts as a stimulus for the enhanced secretion of these ligands. The data generated in this study will allow us to investigate the possible relationship between endorphins and mood state. The practical importance of this research is that the data may allow us to describe a running intensity and duration which optimally stimulates either increased secretion or a reduced metabolic clearance of these ligands. The consequence of this could be that adherence to running programs might be facilitated if they are structured to elicit higher plasma levels of these endogenous opiates.